Silicon-containing heterojunction photovoltaic element and device

ABSTRACT

In one embodiment, a method of forming a photovoltaic device is provided which includes providing an absorption layer comprising a silicon-containing semiconductor layer of a first conductivity type and having a top surface and a bottom surface that opposes the top surface. A front contact is formed on the top surface of the absorption layer, and a back contact is formed on the bottom surface of the absorption layer. The forming of the front contact and the back contact can occur in any order. The back contact that is formed comprises at least one back contact semiconductor material layer of the first conductivity type and having a lower band-offset than that of hydrogenated amorphous silicon with crystalline Si and/or a higher activated doping of the first conductivity type than that of the doped hydrogenated amorphous silicon layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/156,009, filed Jun. 8, 2011 the entire content and disclosure ofwhich is incorporated herein by reference.

BACKGROUND

The present disclosure relates to a photovoltaic device such as, forexample, a solar cell, in which the tunneling barrier for holecollection at either the front contact or the back contact of asilicon-containing heterojunction cell is reduced, without compromisingthe surface passivation either at the front contact or at the backcontact.

A photovoltaic device is a device that converts the energy of incidentphotons to electromotive force (e.m.f.). Typical photovoltaic devicesinclude solar cells, which are configured to convert the energy in theelectromagnetic radiation from the Sun to electric energy. Each photonhas an energy given by the formula E=hv, in which the energy E is equalto the product of the Plank constant h and the frequency v of theelectromagnetic radiation associated with the photon.

A photon having energy greater than the electron binding energy of amatter can interact with the matter and free an electron from thematter. While the probability of interaction of each photon with eachatom is probabilistic, a structure can be built with a sufficientthickness to cause interaction of photons with the structure with highprobability. When an electron is knocked off an atom by a photon, theenergy of the photon is converted to electrostatic energy and kineticenergy of the electron, the atom, and/or the crystal lattice includingthe atom. The electron does not need to have sufficient energy to escapethe ionized atom. In the case of a material having a band structure, theelectron can merely make a transition to a different band in order toabsorb the energy from the photon.

The positive charge of the ionized atom can remain localized on theionized atom, or can be shared in the lattice including the atom. Whenthe positive charge is shared by the entire lattice, thereby becoming anon-localized charge, this charge is described as a hole in a valenceband of the lattice including the atom Likewise, the electron can benon-localized and shared by all atoms in the lattice. This situationoccurs in a semiconductor material, and is referred to asphotogeneration of an electron-hole pair. The formation of electron-holepairs and the efficiency of photogeneration depend on the band structureof the irradiated material and the energy of the photon. In case theirradiated material is a semiconductor material, photogeneration occurswhen the energy of a photon exceeds the band gap energy, i.e., theenergy difference of a band gap of the irradiated material.

The direction of travel of charged particles, i.e., the electrons andholes, in an irradiated material is sufficiently random (known ascarrier “diffusion”). Thus, in the absence of an electric field,photogeneration of electron-hole pairs merely results in heating of theirradiated material. However, an electric field can break the spatialdirection of the travel of the charged particles to harness theelectrons and holes formed by photogeneration.

One exemplary method of providing an electric field is to form a p-n orp-i-n junction around the irradiated material. Due to the higherpotential energy of electrons (corresponding to the lower potentialenergy of holes) in the p-doped material with respect to the n-dopedmaterial, an electric field is generated from the direction of then-doped region toward the p-doped region. Electrons generated in theintrinsic and p-doped regions drift towards the n-doped region due tothe electric field, and holes generated in the intrinsic and n-dopedregions drift towards the p-doped region. Thus, the electron-hole pairsare collected systematically to provide positive charges at the p-dopedregion and negative charges at the n-doped region. The p-n or p-i-njunction forms the core of this type of photovoltaic device, whichprovides electromotive force that can power a device connected to thepositive node at the p-doped region and the negative node at the n-dopedregion.

SUMMARY

A photovoltaic device such as, for example, a solar cell, is provided inwhich the tunneling barrier for hole collection at either the frontcontact or the back contact of a silicon-containing heterojunction cellis reduced. The reduction in the tunneling barrier for hole collectionis obtained without compromising the surface passivation either at thefront contact or at the back contact. In the present disclosure, theintrinsic and/or doped hydrogenated amorphous silicon (a-Si:H) layer(s)at the back contact or at the front contact is replaced with intrinsicand/or doped layer(s) of other semiconductor materials to reduce thetunneling barrier for hole collection and, in turn, enhance theefficiency of the photovoltaic device. This is achieved due to any orall of the following: (i) lower valence band offset between theabsorption layer material and at least one of the contact layermaterials, (ii) higher doping efficiency of at least one of the contactlayer materials, and/or (iii) “transfer doping” (also known as“modulation doping”) of at least one of the contact layer materials. Thereduction of the tunneling barrier for hole collection and increasedefficient is obtained without compromising the passivation of the eitherthe front contact or the back contact.

In one embodiment of the present disclosure, a photovoltaic device isprovided that includes an absorption layer comprising asilicon-containing semiconductor layer of a first conductivity type andhaving a top surface and a bottom surface that opposes the top surface.The photovoltaic device also includes a front contact located on the topsurface of the absorption layer, and a back contact located on thebottom surface of the absorption layer. The back contact comprises atleast one back contact semiconductor material layer of the firstconductivity type and having a lower band-offset than that ofhydrogenated amorphous silicon with crystalline Si, and/or a higheractivated doping of the first conductivity type than that of the dopedhydrogenated amorphous silicon layer. The higher activated doping in theaforementioned back contact materials may be due to a higher dopingefficiency of the material compared to hydrogenated amorphous Si, and/ortransfer doping of the back contact material. Transfer doping of theaforementioned back contact material refers to the transfer of thecarriers corresponding to the first conductivity type from the adjacentsemiconductor layer(s) into the aforementioned back contact material,and/or the transfer of the opposite type of carriers (corresponding tothe second conductivity type) from the back-contact material into theadjacent semiconductor layer(s). Carriers corresponding to n-typeconductivity are electrons, and carriers corresponding to p-typeconductivity are holes. Throughout this disclosure, the secondconductivity type refers to a conductivity type opposite to that of thefirst conductivity type, i.e. if the first conductivity type is p-type,the second conductivity type is n-type and vice versa.

In another embodiment of the present disclosure, a photovoltaic deviceis provided that includes an absorption layer comprising asilicon-containing semiconductor layer of a first conductivity type andhaving a top surface and a bottom surface that opposes the top surface.The photovoltaic device also includes a front contact located on the topsurface of the absorption layer, and a back contact located on thebottom surface of the absorption layer. The front contact of thisphotovoltaic device comprises at least one front contact semiconductormaterial layer of a second conductivity type and having a lowerband-offset than that of hydrogenated amorphous silicon with crystallineSi, and/or a higher activated doping of the second type than that of thedoped hydrogenated amorphous silicon layer. The higher activated dopingin the aforementioned back contact materials may be due to a higherdoping efficiency of the material compared to hydrogenated amorphous Si,and/or transfer doping of the back contact material. Transfer doping ofthe aforementioned back contact material refers to the transfer of thecarriers corresponding to the second conductivity type from the adjacentsemiconductor layer(s) into the aforementioned back contact material,and/or the transfer of the opposite type of carriers (corresponding tothe first conductivity type) from the back-contact material into theadjacent semiconductor layer(s).

The present disclosure also provides methods of forming theaforementioned photovoltaic devices. In one embodiment, the methodincludes providing an absorption layer comprising a silicon-containingsemiconductor layer of a first conductivity type and having a topsurface and a bottom surface that opposes the top surface. A frontcontact is formed on the top surface of the absorption layer, and a backcontact is formed on the bottom surface of the absorption layer. Theforming of the front contact and the back contact can occur in anyorder. The back contact that is formed comprises at least one backcontact semiconductor material layer of the first conductivity type andhaving a lower band-offset than that of hydrogenated amorphous siliconwith crystalline Si and/or a higher activated doping of the firstconductivity type than that of the doped hydrogenated amorphous siliconlayer.

In another embodiment of the present disclosure, the method includesproviding an absorption layer comprising a silicon-containingsemiconductor layer of a first conductivity type and having a topsurface and a bottom surface that opposes the top surface. A frontcontact is formed on the top surface of the absorption layer, and a backcontact is formed on the bottom surface of the absorption layer. Theforming of the front and back contacts may occur in any order. The frontcontact that is formed in this embodiment comprises at least one frontcontact semiconductor material layer of a second conductivity type andhaving a lower band-offset than that of hydrogenated amorphous siliconwith crystalline and/or a higher activated doping of the secondconductivity type than that of the doped hydrogenated amorphous siliconlayer. The second conductivity/doping type is opposite to that of thefirst conductivity/doping type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view)depicting a photovoltaic device in accordance with one embodiment of thepresent disclosure.

FIGS. 2A-2C are pictorial representations (through cross sectionalviews) depicting exemplary front contact structures that can be used inthe photovoltaic device of FIG. 1.

FIGS. 3A-3D are pictorial representations (through cross sectionalviews) depicting other back contact structures of the present disclosurethat can be used in place of the back contact structure illustrated inFIG. 1.

FIG. 4 is a pictorial representation (through a cross section view)depicting a photovoltaic device in accordance with another embodiment ofthe present disclosure.

FIG. 5A-5D are pictorial representations (through cross sectional views)depicting exemplary back contact structures that can be used in thephotovoltaic device of FIG. 4.

FIGS. 6A-6D are pictorial representations (through cross sectionalviews) depicting other front contact structure that can be used in placeof the front contact illustrated in FIG. 4.

FIG. 7A is a pictorial representation (through a cross sectional view)illustrating a photovoltaic structure in accordance with Example 1 ofthe present disclosure.

FIG. 7B is a graph illustrating the experimental output characteristicsof the photovoltaic device of FIG. 7A as measured under a lightintensity of 1 sun.

FIG. 7C is a pictorial representation (through a cross sectional view)illustrating a photovoltaic structure in accordance with Example 1 ofthe present disclosure.

DETAILED DESCRIPTION

The present disclosure, which provides photovoltaic devices withenhanced efficiency, will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that in thedrawings like and corresponding elements are referred to using likereference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present disclosure. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present disclosure may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present disclosure.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

Prior art heterojunction intrinsic thin layer (HIT) solar cells havereached an efficiency up to 23% in laboratory and up to 21% efficiencyin production. The prior art HIT cells are comprised of thin stacks ofintrinsic/doped hydrogenated amorphous silicon (a-Si:H) serving as thefront and back contacts, on an absorption layer composed ofsingle-crystalline silicon (c-Si). The absorption layer of prior art HITcells can have an n-type conductivity or p-type conductivity. However,HIT cells having absorption layers with p-type conductivities have lowerefficiencies (up to 20%) than their counterparts including an absorptionlayer with n-type conductivities. This difference is efficiency ispartly due to the large valence band-offset between a-Si:H and p-typec-Si which hampers the collection of holes at the back contact. Atransparent conductive oxide (TCO) is typically used as the frontcontact electrode, while the back contact electrode is composed of TCOor a metal. Typically, metal fingers are used for improving the lateralcollection of carriers when the contact electrode is composed of TCO.

One problem with the existing HIT solar cells is that the efficiency ishindered by high tunneling barriers for hole collection that exist atthe back contact or the front contact of the HIT cell with c-Siabsorption layers. This due to the large valence band-offset betweencrystalline Si and hydrogenated amorphous Si, and/or the low dopingefficiency of hydrogenated amorphous Si. Although it is possible toreduce the tunneling barrier for hole collection in either the front orback contact layers by reducing the thickness of the intrinsic and/orp+a-Si:H layers within prior art HIT solar cells, such a method cancompromise the passivation, and/or reduce the electric field at the backcontact or the front contact of the HIT cell.

The present disclosure provides photovoltaic devices such as, forexample, solar cells, in which the tunneling barrier for hole collectionat either the front contact or the back contact of a siliconheterojunction cell is reduced, without compromising the surfacepassivation and/or the electric field either at the front contact or atthe back contact.

The aforementioned is achieved in the present disclosure by replacingthe intrinsic and/or doped hydrogenated amorphous silicon (a-Si:H)layer(s) at the back contact or at the front contact with an intrinsicand/or doped layer(s) of a semiconductor material having a lower valenceband-offset than that of a:Si—H with c-Si, and/or a higher activateddoping than that of doped hydrogenated amorphous silicon. This lowersthe tunneling barrier for hole collection and enhances the efficiency ofthe photovoltaic device.

Band offset refers to the discontinuity of the energy bands at aheterojunction. In particular, conduction band offset between twomaterials is the difference between the electron affinities of the twomaterials, and valence band offset between two materials is thedifference between the bandgap difference and the electron affinitydifference of the two materials. The sum of the conduction band offsetand valence band offset therefore equals the bandgap difference of thetwo the materials.

An activated impurity (i.e., dopant) atom refers to an impurity (i.e.,dopant) atom which is accommodated in the host lattice in such a way(typically substitutionally, i.e., sitting on a lattice site,substituting a host atom) that it donates at least one free electron oraccepts at least one free electron (donates at least one free hole) tothe host lattice.

As known in the art, hydrogenated amorphous silicon typically depositedby PECVD or HWCVD (hot-wire CVD) may contain a nano/micro-crystalline,polycrystalline or single-crystalline portion. In particular, when growndirectly on single-crystalline silicon, the initial portion or theentire film may be grown single-crystalline or poly-crystalline,depending on the growth conditions. Throughout this disclosure, theusage of the term “hydrogenated amorphous silicon” covers thepossibility that the film may contain a crystalline portion.

As used herein, a “photovoltaic device” is a device, such as a solarcell, that produces free electrons hole pairs when exposed to radiation,such as light, and results in the production of an electric current. Thephotovoltaic device typically includes layers of p-type conductivity andn-type conductivity that share an interface to provide a junction. The“absorption layer” of the photovoltaic device is the material thatreadily absorbs photons to generate charge carriers, i.e., freeelectrons or holes. A portion of the photovoltaic device, between thefront side and the junction is referred to as the emitter, and thejunction is referred to as the “emitter junction”. The emitterrepresents a portion of the front contact. The emitter portion of thefront contact may be present atop the absorption layer, in which theemitter includes at least one semiconductor layer that has aconductivity type that is opposite the conductivity type of theabsorption layer. The back contact, which may be present below theabsorption layer, has at least one semiconductor layer that has aconductivity type that is the same as the absorption layer.

In one example, when the Sun's energy in the form of photons collects inthe cell layers electron-hole pairs are generated in the material withinthe photovoltaic device. The emitter junction provides the requiredelectric field for the collection of the photogenerated electrons andholes on the p-doped side and n-doped sides of the emitter junction,respectively. For this reason, in this example, at least one p-typelayer of the photovoltaic device may provide the absorption layer, andat least one adjacent n-type layer may provide the front contact. Inanother embodiment, at least one n-type layer of the photovoltaic devicemay provide the absorption layer, and at least one adjacent p-type layermay provide the emitter portion of the front contact.

Reference is first made to FIGS. 1, 2A, 2B, 2C, 3A, 3B, 3C, and 3D whichillustrate an embodiment of the present disclosure in which thetunneling barrier for hole collection is reduced by replacing theintrinsic and/or doped hydrogenated amorphous silicon (a-Si:H) layer(s)at the back contact with an intrinsic and/or doped layer(s) of asemiconductor material having a lower valence band-offset than that ofa:Si—H with c-Si and/or a higher activated doping than that of dopedhydrogenated amorphous silicon.

Specifically, FIG. 1 depicts a photovoltaic device 10 in accordance withone embodiment of the present disclosure. The photovoltaic device 10shown in FIG. 1 includes an absorption layer 12, a back contact 14, anda front contact 24. The photovoltaic device 10 also includes optionalback metal fingers 22 present on a bottommost surface of the backcontact 14, and optional front metal fingers 26 present on an upper mostsurface of the front contact 24. In the photovoltaic devices of thepresent disclosure, the front contact is located above the absorptionlayer, and the back contact is located beneath the absorption layer.Thus, photovoltaic devices of this disclosure comprise, from top tobottom, a front contact, an absorption layer and a back contact. In thepresent disclosure the order of forming the front and back contacts mayvary and is not critical to the practice of the various embodiments ofthe present disclosure.

The absorption layer 12 employed in this embodiment of the presentdisclosure comprises a Si-containing semiconductor layer having a firstconductivity type dopant present therein. In one embodiment, the firstconductivity dopant is a p-type dopant, and therefore the absorptionlayer 12 comprises a p-type Si-containing semiconductor layer. Inanother embodiment, the first conductivity dopant is an n-type dopant,and therefore the absorption layer 12 comprises an n-type Si-containingsemiconductor layer.

As used throughout the present application, “p-type” refers the additionof impurities to an intrinsic semiconductor that creates deficiencies ofvalence electrons. In a Si-containing absorption layer 12, examples ofp-type dopants, i.e., impurities, include but are not limited to, boron,aluminum, gallium and indium. In one embodiment, in which the firstconductivity type of the Si-containing semiconductor material of theabsorption layer 12 is p-type, the p-type dopant is present in aconcentration ranging from 1×10⁹ atoms/cm³ to 1×10²° atoms/cm³. Inanother embodiment, in which the first conductivity type is p-type, thep-type dopant is present in a concentration ranging from 1×10¹⁴atoms/cm³ to 1×10¹⁹ atoms/cm³. As used throughout the presentapplication, “n-type” refers to the addition of impurities thatcontributes free electrons to an intrinsic semiconductor. In aSi-containing absorption layer 12, examples of n-type dopants, i.e.,impurities, include but are not limited to, antimony, arsenic andphosphorous. In one embodiment, in which the first conductivity type ofthe Si-containing semiconductor material of the absorption layer 12 isn-type, the n-type dopant is present in a concentration ranging from1×10⁹ atoms/cm³ to 1×10²° atoms/cm³. In another embodiment, in which thefirst conductivity type is n-type, the n-type dopant is present in aconcentration ranging from 1×10¹⁴ atoms/cm³ to 1×10¹⁹

In one embodiment, the absorption layer 12 can be a single crystallineSi-containing semiconductor material. The term “single crystalline”denotes a crystalline solid, in which the crystal lattice of the entiresample is substantially continuous and substantially unbroken to theedges of the sample, with substantially no grain boundaries. In anotherembodiment, the absorption layer 12 can be a multicrystallinesemiconductor material. A multicrystalline semiconductor materialtypically includes large grains of crystalline semiconductor material.

In one embodiment, the absorption layer 12 is composed of at least oneof Si, SiGe, SiC, and SiGeC. Typically, the Si-containing semiconductormaterial of the absorption layer 12 comprises single-crystalline Si. Inone embodiment, the Si-containing semiconductor material of theabsorption layer 12 comprises multicrystalline Si. In anotherembodiment, the absorption layer 12 is p-type single crystalline Si.

The absorption layer 12 typically has a thickness ranging from 5 nm to 5microns, with a thickness from 50 nm to 2 microns being more typical.Other thicknesses which can be greater than or less than the thicknessrange mentioned hereinabove can also be employed. The band gap of theabsorption layer 12 is typically from 0.6 eV to 1.2 eV, with a band gapfrom 0.8 eV to 1.1 eV being more typical.

The optional back metal fingers 22 and optional front metal fingers 26shown in the drawings may be deposited using a screen printingtechnique. In another embodiment, the optional back metal fingers 22 andoptional front metal fingers 26 are provided by the application of anetched or electroformed metal pattern. The metallic material used informing the metal pattern for the optional back metal fingers 22 andoptional front metal fingers 26 may include applying a metallic paste.The metallic paste may be any conductive paste, such as Al paste, Agpaste or AlAg paste. The metallic material used in forming the metalpattern for the optional back metal fingers 22 and optional front metalfingers 26 may also be deposited using sputtering, thermal or e-beamevaporation, or plating. The thickness of metal fingers 22 and 26 mayvary depending on the type of process used in forming the same.Typically, the metal fingers 22 and 26 have a thickness from 100 nm to15 μm, with a thickness from 1 μm to 10 μm being more typical.

The order of forming the front contact 24 and the back contact 14 inthis embodiment of the present disclosure is not critical. For example,the front contact 24 can be formed first and then the back contact 14can be formed. In another embodiment, the back contact 14 can be formedfirst and then the front contact 24 can be formed.

The photovoltaic device shown in FIG. 1 also includes a front contact 24(i.e., emitter contact) which is located on a top surface of theabsorption layer 12. In this embodiment of the present disclosure, thestructure of the front contact 24 includes any conventional frontcontact structure that can be employed in photovoltaic devices. FIGS.2A-2C illustrate some exemplary front contact structures that can beemployed as the front contact 24 in the photovoltaic device 10 ofFIG. 1. The top metal fingers 26 of FIG. 1 are duplicated in FIGS.2A-2C.

Specifically, FIG. 2A illustrates a standard heterojunction contactstructure that includes a front contact intrinsic amorphoussemiconductor material layer 28 which is in direct contact with the topsurface of the absorption layer 12. The term “intrinsic semiconductor”,also called an undoped semiconductor or i-type semiconductor, is asubstantially pure semiconductor without any significant dopant speciespresent. The number of charge carriers in the intrinsic semiconductor isdetermined by the properties of the material itself instead of theamount of impurities, i.e., dopants. Typically, in intrinsicsemiconductors the number of excited electrons and the number of holesare equal (n=p). The front contact intrinsic amorphous semiconductormaterial layer 28 can serve to passivate the top surface of theabsorption layer 12, and reduce electron-hole recombination. The frontcontact intrinsic amorphous semiconductor material layer 28 istypically, but not necessarily always hydrogenated. Typically, the frontcontact intrinsic amorphous semiconductor material layer 28 is composedof intrinsic hydrogenated amorphous silicon (i a-Si:H). Typically, thethickness of the front contact intrinsic amorphous semiconductormaterial layer 28 is from 2 nm to 15 nm, although lesser and greaterthicknesses can also be employed.

The front contact intrinsic amorphous semiconductor material layer 28 isformed utilizing any chemical or physical growth process including anysemiconductor precursor source material. In some embodiments, theintrinsic hydrogenated semiconductor containing material used in forminglayer 28 is deposited in a process chamber containing a semiconductorprecursor source gas and a carrier gas including hydrogen. Hydrogenatoms in the precursor source gas or in the hydrogen gas within thecarrier gas are incorporated into the deposited material to form thefront contact intrinsic amorphous semiconductor material layer 28. Thefront contact intrinsic amorphous semiconductor material layer 28 isoptional, and may be omitted.

The standard heterojunction contact structure shown in FIG. 2A alsoincludes a front contact amorphous semiconductor layer 30 that has asecond conductivity type that is different from the first conductivitytype of the Si-containing semiconductor material employed as theabsorption layer 12. Thus, when the absorption layer 12 comprises ap-type Si-containing semiconductor material, the front contact amorphoussemiconductor layer 30 comprises n-type doping. In such an embodiment,the front contact amorphous semiconductor layer 30 has an n-type dopantconcentration ranging from 10¹⁶ atoms/cm³ to 10²¹ atoms/cm³, with therange of 10¹⁸ atoms/cm³ to 10²⁰ atoms/cm³ being more typical. The dopingefficiency (i.e., the ratio of activated dopant atoms to the totaldopant atoms) in layer 30 typically ranges from 0.1% to 20% althoughhigher and lower doping efficiencies are possible. Typically, the dopingefficiency is decreased by increasing the dopant atom concentrationLikewise, when the absorption layer 12 comprises an n-type Si-containingsemiconductor material, the front contact amorphous semiconductor layer30 comprises p-type doping. In such an embodiment, the front contactamorphous semiconductor layer 30 has a p-type dopant concentrationranging from 10¹⁶ atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸atoms/cm³ to 10²⁰ atoms/cm³ being more typical. The doping efficiency(i.e., the ratio of activated dopant atoms to the total dopant atoms) inlayer 30 typically ranges from 0.1% to 20% although higher and lowerdoping efficiencies are possible. Typically, the doping efficiency isdecreased by increasing the dopant atom concentration.

In some embodiments, the front contact amorphous semiconductor layer 30can be in direct contact with the top surface of the absorption layer12. In another embodiment, the front contact intrinsic amorphoussemiconductor material layer 28 is positioned between the front contactamorphous semiconductor layer 30 and the top surface of the absorptionlayer 12. The front contact amorphous semiconductor layer 30 can includeone of the Si-containing semiconductor materials mentioned above for theabsorption layer 12. In one embodiment, the front contact amorphoussemiconductor layer 30 is comprised of the same Si-containingsemiconductor material as that of the absorption layer 12. For example,the front contact amorphous semiconductor layer 30 and the absorptionlayer 12 can be both comprised of silicon.

The front contact amorphous semiconductor layer 30 can be formedutilizing any physical or chemical growth process that is well known tothose skilled in the art. In one embodiment, the physical or chemicalgrowth process includes an in-situ doped growth process in which thedopant atom is introduced with the semiconductor precursor sourcematerial, e.g., silane, during the formation of the front contactamorphous semiconductor layer 30.

In some embodiments, the front contact amorphous semiconductor layer 30can be comprised of a hydrogenated amorphous semiconductor material. Thehydrogenated amorphous semiconductor material that can be used as layer30 can be deposited in a process chamber containing a semiconductorprecursor source material gas and a carrier gas, which may containhydrogen. To facilitate incorporation of hydrogen in the hydrogenatedsemiconductor-containing material, a carrier gas including hydrogen canbe employed. Hydrogen atoms in the precursor gas and/or the carrier gasare incorporated into the deposited material to form a hydrogenatedamorphous semiconductor-containing material that can be used as layer30.

The thickness of the front contact amorphous semiconductor layer 30 canvary depending on the conditions of the growth process employed, as wellas the duration of growth. Typically, the front contact amorphoussemiconductor layer 30 has a thickness from 3 nm to 30 nm.

The standard heterojunction contact structure shown in FIG. 2A furtherincludes a front contact transparent conductive material layer 32 thatis located on an upper surface of the front contact amorphoussemiconductor layer 30. Throughout this disclosure, an element is“transparent” if the element is sufficiently transparent in the visibleelectromagnetic spectral range. The front contact transparent conductivematerial layer 32 includes a conductive material that is transparent inthe range of electromagnetic radiation at which photogeneration ofelectrons and holes occur within the solar cell structure. In oneembodiment, the front contact transparent conductive material layer 32can include a transparent conductive oxide such as, but not limited to,a fluorine-doped tin oxide (SnO₂:F), an aluminum-doped zinc oxide(ZnO:Al), tin oxide (SnO) and indium tin oxide (InSnO₂, or ITO forshort). The thickness of the front contact transparent conductivematerial layer 32 may vary depending on the type of transparentconductive material employed, as well as the technique that was used informing the transparent conductive material. Typically, and in oneembodiment, the thickness of the front contact transparent conductivematerial layer 32 ranges from 20 nm to 500 nm. Other thicknesses,including those less than 20 nm and/or greater than 500 nm can also beemployed. The optimum thickness of TCO for minimizing reflection fromthe surface of Si is in the range of 70 nm to 110 nm. The front contacttransparent conductive material layer 32 is typically formed using adeposition process, such as sputtering or CVD. Examples of CVD processessuitable for forming the front contact transparent conductive materiallayer 32 include, but are not limited to, APCVD, LPCVD, PECVD, MOCVD andcombinations thereof. Examples of sputtering include, but are notlimited to, RF and DC magnetron sputtering.

The top, bottom, or both surfaces of the absorption layer 12, and/or thetop surface of the transparent conductive material layer 32 may betextured. A textured (i.e., specially roughened) surface is used insolar cell applications to increase the efficiency of light absorption.The textured surface decreases the fraction of incident light lost toreflection relative to the fraction of incident light transmitted intothe cell since photons incident on the side of an angled feature will bereflected onto the sides of adjacent angled features and thus haveanother chance to be absorbed. Moreover, the textured surface increasesinternal absorption, since light incident on an angled surface willtypically be deflected to propagate through the device at an obliqueangle, thereby increasing the length of the path taken to reach thedevice's back surface, as well as making it more likely that photonsreflected from the device's back surface will impinge on the frontsurface at angles compatible with total internal reflection and lighttrapping. In one embodiment, the texturing is achieved utilizing ahydrogen based wet etch chemistry, such as, for example, etching in HCl.In some embodiments, the textured upper surface can be achieved duringformation, i.e., deposition, of the front contact transparent conductivematerial layer 32. In one embodiment, the texturing of thesingle-crystalline Si absorbing layer 10 is achieved utilizing a KOHbased wet etch chemistry to realize random pyramids, or invertedpyramids.

The optional front metal fingers 26 can be formed atop the front contacttransparent conductive material layer 32 utilizing one of the techniquesmentioned above.

FIG. 2B illustrates a standard single emitter structure that can beemployed as the front contact 24. The standard single emitter includes asemiconductor material layer 34 of a second conductivity type that isopposite the first conductivity type of the Si-containing semiconductormaterial employed as the absorption layer 12. Thus, when the absorptionlayer 12 comprises a p-type Si-containing semiconductor material, thesemiconductor material layer 34 comprises n-type doping. In such anembodiment the semiconductor material layer 34 has an n-type dopantconcentration ranging from 10¹⁶ atoms/cm³ to 5×10²⁰ atoms/cm³, with therange of 10¹⁸ atoms/cm³ to 5×10¹⁹ atoms/cm³ being more typical Likewise,when the absorption layer 12 comprises an n-type Si-containingsemiconductor material, the semiconductor material layer 34 comprisesp-type doping. In such an embodiment the semiconductor material layer 34has a p-type dopant concentration ranging from 10¹⁶ atoms/cm³ to 5×10²°atoms/cm³, with the range of 10¹⁸ atoms/cm³ to 5×10¹⁹ atoms/cm³ beingmore typical.

The semiconductor material layer 34, which is in direct contact with thetop surface of the absorption layer 12, can comprise the same ordifferent semiconductor material as that of the absorption layer 12. Inone embodiment, the semiconductor material layer 34 and the absorptionlayer 12 are both comprised of Si. The semiconductor material layer 34can be single crystalline or a microcrystalline semiconductor material.

The standard single emitter structure shown in FIG. 2B also includes apatterned dielectric material 36 located on an upper surface of thesemiconductor material layer 34. The patterned dielectric material 36can include any insulator material including, for example, an oxide, anitride and/or an oxynitride. In one embodiment, the patterneddielectric material 36 is comprised of a semiconductor oxide, asemiconductor nitride and/or a semiconductor oxynitride. In someembodiments, the patterned dielectric material 36 can haveanti-reflective properties. In such an instance, the patterneddielectric material 36 can comprise a dual layer structure composed ofzinc-sulfide (ZnS) and magnesium fluoride (MgF₂), on top of the oxide,nitride or oxynitride layer.

The patterned dielectric material 36 can be formed by first depositing ablanket layer of dielectric material on the upper surface of thesemiconductor material layer 34 utilizing any deposition processincluding, for example, chemical vapor deposition, plasma enhancedchemical vapor deposition, atomic layer deposition and chemical solutiondeposition. The blanket layer of dielectric material may be also grownby a chemical reaction consuming part of the semiconductor layer 34,such as dry or wet oxidation. Following deposition of the blanketdielectric material layer, the dielectric material layer can bepatterned by lithography and etching. Lithography includes applying aphotoresist to the upper surface of the material layer to be patterned,exposing the photoresist to a desired pattern of radiation anddeveloping the resist pattern utilizing a conventional developer. Theetching that can be used to pattern the blanket dielectric materiallayer can include dry etching (i.e., reactive ion etching, plasmaetching, ion beam etching or laser ablation) or wet etching.

The metal fingers 26 are then formed as described above into theopenings within the patterned dielectric material 36 as shown in FIG.2B. In this embodiment, a lower portion of the metal fingers within theopening contacts an upper surface of the semiconductor material layer34.

FIG. 2C illustrates a standard double emitter contact structure that canbe used as front contact 24. The double emitter contact structure shownin FIG. 2C is similar to the single emitter structure shown in FIG. 2Bexcept that highly doped regions 38 are present at the contact areaswith the fingers. Highly doped regions 38 may be formed in thesemiconductor material layer 34 utilizing the patterned dielectricmaterial 36 as an ion implantation mask followed by a diffusiondrive-in. The highly doped regions 38 include the same conductivity typedopant as the semiconductor material layer 34. However, theconcentration of the dopant within the highly doped regions 38 isgreater than the dopant that is present in the semiconductor materiallayer 34. When the highly doped regions 38 and the semiconductormaterial layer 34 both include an n-type dopant, the n-type dopantconcentration in the highly doped regions 38 can range from 10¹⁸atoms/cm³ to 5×10²° atoms/cm³ Likewise, when the highly doped regions 38and the semiconductor material layer 34 both include a p-type dopant,the p-type dopant concentration in the highly doped regions 38 can rangefrom 10¹⁸ atoms/cm³ to 5×10²° atoms/cm³.

The contact structure shown in FIG. 2C is formed utilizing the samebasic processing steps mentioned above for forming the single emitterstructure shown in FIG. 2B except that prior to forming metal fingers26, a second conductivity type dopant is introduced into thesemiconductor material layer 34 for example by ion implantation or gasphase diffusion utilizing the patterned dielectric material 36 as an ionimplantation mask. In the case of ion implantation, a diffusion annealis performed subsequently, utilizing standard diffusion annealtemperatures that are well known to those skilled in the art.

Referring back to FIG. 1, the back contact 14 of the illustratedphotovoltaic device includes an optional back contact intrinsicamorphous semiconductor material layer 16 which is in direct contactwith the bottom surface of the absorption layer 12. Again the term“intrinsic semiconductor” denotes a substantially pure semiconductorwithout any significant dopant species present. The number of chargecarriers in the intrinsic semiconductor is determined by the propertiesof the material itself instead of the amount of impurities, i.e.,dopants. Typically, in intrinsic semiconductors the number of excitedelectrons and the number of holes are equal (n=p). The back contactintrinsic amorphous semiconductor material layer 16 can serve topassivate the bottom surface of the absorption layer 12, and reduceelectron-hole recombination. The back contact intrinsic amorphoussemiconductor material layer 16 is typically, but not necessarily alwayshydrogenated. Typically, the back contact intrinsic amorphoussemiconductor material layer 16 is composed of intrinsic hydrogenatedamorphous silicon (i a-Si:H), hydrogenated amorphous Ge (a-Ge:H), orhydrogenated amorphous silicon germanium alloy (a-SiGe:H).

In some cases, the back contact intrinsic amorphous semiconductormaterial layer 16 comprises a same semiconductor material as theabsorption layer 12. In other embodiments, the back contact intrinsicamorphous semiconductor material layer 16 has a lower valenceband-offset than that of the absorption layer 12.

In cases in which the intrinsic layer includes SiGe, the Si (or Ge)content of the SiGe alloy may vary in a gradient fashion which caneither increase or decrease from a surface of layer 16 that is nearestto the absorption layer 12 to a surface of layer 16 that is furthestfrom the absorption layer 12. The back contact intrinsic amorphoussemiconductor material layer 16 can also include carbon, fluorine,deuterium, oxygen and/or nitrogen, which can be added either during orafter formation of the back contact intrinsic amorphous semiconductormaterial layer 16.

Typically, the thickness of the back contact intrinsic amorphoussemiconductor material layer 16 is from 2 nm to 25 nm, although lesserand greater thicknesses can also be employed.

The back contact intrinsic amorphous semiconductor material layer 16 isformed utilizing any chemical or physical growth process including anysemiconductor precursor source material. In some embodiments, theintrinsic hydrogenated semiconductor containing material used in forminglayer 16 is deposited in a process chamber containing a semiconductorprecursor source gas and a carrier gas which may include hydrogen.Hydrogen atoms in the precursor gas or in the hydrogen gas within thecarrier gas are incorporated into the deposited material to form theback contact intrinsic amorphous semiconductor material layer 16. Theback contact intrinsic amorphous semiconductor material layer 16 isoptional, and may be omitted.

The back contact 14 shown in FIG. 1 also includes a back contactsemiconductor material layer 18 of the first conductivity type locatedeither directly on the bottom surface of the absorption layer 12 or on abottom surface of the back contact intrinsic amorphous semiconductormaterial layer 16. The back contact semiconductor material layer 18employed in this embodiment of the present disclosure has a lowervalence band-offset than that of hydrogenated amorphous silicon withcrystalline silicon, and/or a higher activated doping than that of dopedhydrogenated amorphous silicon.

In one embodiment, and when the absorption layer comprises Si, the backcontact semiconductor material layer 18 can be comprised of Ge or a SiGealloy. When a SiGe alloy is employed as the back contact semiconductormaterial layer 18, the Si (or Ge) content may be constant throughout theentire thickness of the back contact semiconductor material layer 18. Inother embodiments, the Si (or Ge) content of the SiGe alloy may vary ina gradient fashion which can for example increase or decrease from asurface of the back contact semiconductor material layer 18 that isnearest to the absorption layer 12 to a surface of the back contactsemiconductor material layer 18 that is furthest from the absorptionlayer 12.

The back contact semiconductor material layer 18 can be amorphous,nanocrystalline (i.e., microcrystalline) or polycrystalline. In someembodiments, back contact semiconductor material layer 18 can behydrogenated. In other embodiments, the back contact semiconductormaterial layer 18 is not hydrogenated.

As inferred to above, the back contact semiconductor material layer 18and the absorption layer 12 are of the same conductivity type. However,the first conductivity type dopant present in the back contactsemiconductor material layer 18 is greater than the first conductivitytype dopant present in the absorption layer 12. When the absorptionlayer 12 includes an n-type dopant, the n-type dopant concentration inthe back contact semiconductor material layer 18 can range from 10¹⁶atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸ atoms/cm³ to 10²⁰atoms/cm³ being more typical. The doping efficiency (i.e., the ratio ofactivated dopant atoms to the total dopant atoms) in layer 18 typicallyranges from 0.1% to 20% although higher and lower doping efficienciesare possible. Typically, the doping efficiency is decreased byincreasing the dopant atom concentration. Likewise, when the absorptionlayer 12 includes a p-type dopant, the p-type dopant concentration inthe back contact semiconductor material layer 18 can range from 10¹⁶atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸ atoms/cm³ to 10²⁰atoms/cm³ being more typical. The doping efficiency (i.e., the ratio ofactivated dopant atoms to the total dopant atoms) in layer 18 typicallyranges from 0.1% to 20% although higher and lower doping efficienciesare possible. Typically, the doping efficiency is decreased byincreasing the dopant atom concentration. The back contact semiconductormaterial layer 18 can also include carbon, fluorine, deuterium, oxygenand/or nitrogen, which can be added either during or after formation ofthe back contact semiconductor layer 18.

The back contact semiconductor material layer 18 can be formed utilizingany physical or chemical growth process that is well known to thoseskilled in the art. In one embodiment, the growth process includes anin-situ doped epitaxial growth process in which the dopant atom isintroduced with the semiconductor precursor source material, e.g., asilane, during the formation of the back contact semiconductor materiallayer 18. In another embodiment, a growth process is used to form anundoped semiconductor layer, and thereafter the dopant can be introducedusing one of ion implantation, gas phase doping, liquid solutionspray/mist doping, and/or out-diffusion of a dopant atom from anoverlying sacrificial dopant material layer that can be formed on theundoped semiconductor material, and removed after the out-diffusionprocess.

In some embodiments, the back contact semiconductor material layer 18can be comprised of a hydrogenated semiconductor material. Thehydrogenated semiconductor material that can be used as layer 18 can bedeposited in a process chamber containing a semiconductor precursorsource material gas and a carrier gas which may contain hydrogen.Hydrogen atoms in the precursor and/or carrier gas are incorporated intothe deposited material to form a hydrogenated semiconductor-containingmaterial that can be used as layer 18.

The back contact 14 shown in FIG. 1 also includes a back contacttransparent conductive material layer 20 that is located beneath theback contact semiconductor material layer 18. The back contacttransparent conductive material layer 20 can include one of theconductive materials, have a textured surface and have a thickness asmentioned above for the front contact transparent conductive materiallayer 32 shown in FIG. 2A. The back contact transparent conductivematerial layer 20 can be made using one of the techniques mentionedabove in forming the front contact transparent conductive material layer32. Alternatively, a metal layer may be used instead of the transparentconductive material, layer 20. Typically, if layer 20 is a transparentconductive material, metal fingers 22 are required, while if layer 20 isa metal, metal fingers 22 are not used. The usage of a transparentconductive material facilitates the bifacial operation of the solarcell, if desired.

The optional back contact metal fingers 22 can be formed beneath backcontact transparent conductive material layer 20 utilizing one of thetechniques mentioned above.

Reference is now made to FIGS. 3A-3D which depict other back contactstructures of the present disclosure that can be used in place of theback contact structure illustrated in FIG. 1. FIG. 3A shows anembodiment in which no back contact intrinsic amorphous semiconductormaterial layer 16 is present.

FIG. 3B illustrates an embodiment in which a back contact hydrogenatedamorphous semiconductor material layer 15 of the first conductivity typeis positioned between layers 16 and 18. Thus, when the absorption layer12 comprises a p-type semiconductor material, the back contacthydrogenated amorphous semiconductor material layer 15 comprises p-typedoping. In such an embodiment, the back contact hydrogenated amorphoussemiconductor material layer 15 has a p-type dopant concentrationranging from 10¹⁶ atoms/cm³ to ato10²¹ atoms/cm³, with the range of 10¹⁸atoms/cm³ to 10²⁰ atoms/cm³ being more typical. The doping efficiency(i.e., the ratio of activated dopant atoms to the total dopant atoms) inlayer 15 typically ranges from 0.1% to 20% although higher and lowerdoping efficiencies are possible. Typically, the doping efficiency isdecreased by increasing the dopant atom concentration. Likewise, whenthe absorption layer 12 comprises an n-type semiconductor material, theback contact hydrogenated amorphous semiconductor material layer 15comprises n-type doping. In such an embodiment, the back contacthydrogenated amorphous semiconductor material layer 15 has an n-typedopant concentration ranging from 10¹⁶ atoms/cm³ to 10²¹ atoms/cm³, withthe range of 10¹⁸ atoms/cm³ to 10²⁰ atoms/cm³ being more typical. Thedoping efficiency (i.e., the ratio of activated dopant atoms to thetotal dopant atoms) in layer 15 typically ranges from 0.1% to 20%although higher and lower doping efficiencies are possible. Typically,the doping efficiency is decreased by increasing the dopant atomconcentration.

The back contact hydrogenated amorphous semiconductor material layer 15may or may not comprise the same semiconductor material as that of theabsorption layer 12. In one embodiment, when the absorption layer 12comprises Si, the back contact hydrogenated amorphous semiconductormaterial layer 15 comprises hydrogenated amorphous Si, and may containcarbon and/or germanium. The concentration of carbon and/or germaniummay be constant or vary across the layer.

The back contact hydrogenated amorphous semiconductor material layer 15can be deposited in a process chamber containing a semiconductorprecursor source material gas, first conductivity type dopant and acarrier gas, which may contain hydrogen. Hydrogen atoms in the precursorand/or carrier gas are incorporated into the deposited material.

The thickness of the back contact hydrogenated amorphous semiconductormaterial layer 15 may vary. Typically, the back contact hydrogenatedamorphous semiconductor material layer 15 has a thickness from 3 nm to30 nm.

FIGS. 3C and 3D show an embodiment in which back contact semiconductormaterial layer includes a first back contact semiconductor materiallayer 18′ and a second back contact semiconductor material layer 18″. Inthese embodiments, the first and second back contact semiconductormaterial layer 18′, 18″ are comprised of different semiconductormaterials that both have the lower valence band-offset than that ofhydrogenated amorphous silicon with crystalline silicon, and/or higheractivated doping levels than that of doped hydrogenated amorphoussilicon. In one embodiment, the first back contact semiconductormaterial layer 18′ is comprised of hydrogenated amorphous Ge and thesecond back contact semiconductor material layer 18″ is comprised ofhydrogenated amorphous SiGe.

It is noted that in each of FIGS. 1, 3A, 3B, 3C and 3D, the lowerband-offset semiconductor material within the back contact structureimproves the collection of holes at the back contact by reducing thetunneling barrier for holes.

Reference is now made to FIGS. 4, 5A, 5B, 5C, 5D, 6A, 6B, 6D, 6E whichillustrate an embodiment of the present disclosure in which thetunneling barrier for hole collection is reduced by replacing theintrinsic and/or doped hydrogenated amorphous silicon (a-Si:H) layer(s)at the front contact with an intrinsic and/or doped layer(s) of asemiconductor material having a lower valence band-offset than that ofa:Si—H with c-Si as the absorption layer, and/or higher activated dopinglevel(s) than that of doped hydrogenated amorphous silicon.

Specifically, FIG. 4 depicts a photovoltaic device 50 in accordance withanother embodiment of the present disclosure. The photovoltaic device 50shown in FIG. 4 includes an absorption layer 52, a back contact 64, anda front contact 54. The photovoltaic device also includes optional backmetal fingers 66 present on a bottommost surface of the back contact 64,and optional front metal fingers 62 present on an upper most surface ofthe front contact 54. It is again noted that the order of forming theback contact and the front contact may vary.

In this embodiment, the absorption layer 52 is the same as thatmentioned above for absorption layer 12. As such, the semiconductormaterial, doping concentration, thickness and methods of formingabsorption layer 12 are applicable here for forming absorption layer 52.In one embodiment, absorption layer 52 is n-type crystalline Si.

Also, in this embodiment, the optional front metal fingers 62 and theoptional back metal fingers 66 employed are the same as that mentionedabove for optional front metal fingers 22 and the optional back metalfingers 26. As such, the materials and processes mentioned above forforming the optional front and back metal fingers 22, 26 can be usedhere in this embodiment for forming optional front and back metalfingers 62, 66.

The back contact 64 of the photovoltaic device 50 illustrated in FIG. 4may include any conventional back contact structure that is well knownto those skilled in the art. FIGS. 5A, 5B, 5C and 5D are some exemplaryback contact structures that can be used as back contact 64 shown in theFIG. 4. The back fingers 66 are duplicated in the back contactstructures shown in FIGS. 5A-5D.

Specifically, FIG. 5A illustrates a standard back contactheterostructure which includes an optional back contact intrinsicamorphous semiconductor material layer 72 located on a bottom surface ofthe adsorption layer 52. In this embodiment, the back contact intrinsicamorphous semiconductor material layer 72 comprises the same material asthat of the absorption layer 52. The optional back contact intrinsicamorphous semiconductor material layer 72 can serve to passivate thebottom surface of the absorption layer 52, and reduce electron-holerecombination. The optional back contact intrinsic amorphoussemiconductor material layer 72 is typically, but not necessarily alwayshydrogenated. Typically, the thickness of the optional back contactintrinsic amorphous semiconductor material layer 72 is from 3 nm to 30nm, although lesser and greater thicknesses can also be employed.

The optional back contact intrinsic amorphous semiconductor materiallayer 72 can be formed utilizing any physical or chemical growth processincluding any semiconductor precursor source material. In someembodiments, the intrinsic hydrogenated semiconductor containingmaterial used in forming layer 72 is deposited in a process chambercontaining a semiconductor precursor source gas and a carrier gas whichmay include hydrogen. Hydrogen atoms in the precursor and/or carrier gasare incorporated into the deposited material to form the optional backcontact intrinsic amorphous semiconductor material layer 72.

The back contact structure shown in FIG. 5A also includes a back contactamorphous semiconductor layer 70 that has a first conductivity type thatis same as the first conductivity type of the Si-containingsemiconductor material employed as the absorption layer 52. Thus, whenthe absorption layer 52 comprises a p-type Si-containing semiconductormaterial, the back contact amorphous semiconductor layer 70 comprisesp-type doping. In such an embodiment, the back contact amorphoussemiconductor layer 70 has a p-type dopant concentration ranging from10¹⁶ atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸ atoms/cm³ to10²⁰ atoms/cm³ being more typical. The doping efficiency (i.e., theratio of activated dopant atoms to the total dopant atoms) in layer 70typically ranges from 0.1% to 20% although higher and lower dopingefficiencies are possible. Typically, the doping efficiency is decreasedby increasing the dopant atom concentration. Likewise, when theabsorption layer 52 comprises an n-type Si-containing semiconductormaterial, the back contact amorphous semiconductor layer 70 comprisesn-type doping. In such an embodiment the back contact amorphoussemiconductor layer 70 has an n-type dopant concentration ranging from10¹⁶ atoms/cm³ to 10²¹ atoms/cm³, with the range of 10¹⁸ atoms/cm³ to10²⁰ atoms/cm³ being more typical. The doping efficiency (i.e., theratio of activated dopant atoms to the total dopant atoms) in layer 70typically ranges from 0.1% to 20% although higher and lower dopingefficiencies are possible. Typically, the doping efficiency is decreasedby increasing the dopant atom concentration.

The back contact amorphous semiconductor layer 70 can include one of thesemiconductor materials mentioned above for the absorption layer 52. Inone embodiment, the back contact amorphous semiconductor layer 70 iscomprised of the same semiconductor material as that of the absorptionlayer 52. For example, the back contact amorphous semiconductor layer 70and the absorption layer 52 can both be comprised of Si.

The back contact amorphous semiconductor layer 70 can be formedutilizing any physical or chemical growth process that is well known tothose skilled in the art. In one embodiment, the growth process includesan in-situ doped epitaxial growth process in which the dopant atom isintroduced with the semiconductor precursor source material, e.g., asilane, during the formation of the back contact amorphous semiconductorlayer 70.

In some embodiments, the back contact amorphous semiconductor layer 70can be comprised of a hydrogenated amorphous semiconductor material. Thehydrogenated amorphous semiconductor material that can be used as layer70 can be deposited in a process chamber containing a semiconductorprecursor source material gas and a carrier gas, which may containhydrogen. Hydrogen atoms in the precursor and/or carrier gas areincorporated into the deposited material to form a hydrogenatedamorphous semiconductor-containing material that can be used as layer70.

The thickness of the back contact amorphous semiconductor layer 70 canvary depending on the conditions and duration of the growth processemployed. Typically, the back contact amorphous semiconductor layer 70has a thickness from 3 nm to 30 nm.

The back contact structure shown in FIG. 5A also includes a back contacttransparent conductive material layer 68 located beneath layer 70. Theback contact transparent conductive material layer 68 of this embodimentcan include one of the conductive materials mentioned above for thefront contact transparent conductive material layer 32. The back contacttransparent conductive material layer 68 of this embodiment can also beformed utilizing one of the techniques mentioned above for the frontcontact transparent conductive material layer 32, and the thickness ofthe back contact transparent conductive material layer 68 can be withinthe range provided above for the front contact transparent conductivematerial layer 32.

FIG. 5B illustrates another back contact structure that can be employedwith the photovoltaic device 50 shown in FIG. 4. The back contactstructure shown in FIG. 5B, which represents a blanket back surfacefield region, includes a back contact semiconductor material layer 74having the first conductivity type located beneath and in contact with abottom surface of absorption layer 52. The back contact semiconductormaterial layer 74 having the first conductivity type typically includesthe same semiconductor material as that of the absorption layer 52.Thus, and in one embodiment, the back contact semiconductor materiallayer 74 having the first conductivity type and the absorption layer 52are both comprised of single crystalline or multicrystalline Si.

As mentioned above, the back contact semiconductor material layer 74 andthe absorption layer 52 are both of the first conductivity type. Thus,when the absorptive layer 52 comprises a p-type dopant, the back contactsemiconductor material layer 74 comprises p-type doping. In such anembodiment, the back contact semiconductor material layer 74 has ap-type dopant concentration ranging from 10¹⁶ atoms/cm³ to 5×10²°atoms/cm³. Likewise, when the absorption layer 52 comprises an n-typesemiconductor material, the back contact semiconductor material layer 74comprises n-type doping. In such an embodiment, the back contactsemiconductor material layer 74 has an n-type dopant concentrationranging from 10¹⁶ atoms/cm³ to 5×10²° atoms/cm³.

The back contact semiconductor material layer 74 can be formed utilizingany process well known to those skilled in the art. In one embodiment,the process includes an in-situ doped epitaxial growth process in whichthe dopant atom is introduced with the semiconductor precursor sourcematerial, e.g., a silane, during the formation of the back contactsemiconductor material layer 74. In another embodiment, the dopant canbe introduced into the absorption layer 52 to form the back contactlayer 74 using one of ion implantation, gas phase doping, liquidsolution spray/mist doping, and/or out-diffusion of a dopant atom froman overlying sacrificial dopant material layer that can be formed on theundoped semiconductor material, and removed after the out-diffusionprocess.

The thickness of the back contact semiconductor material layer 74 mayvary depending on the exact conditions used in forming the layer.Typically, the back contact semiconductor material layer 74 has athickness from 1 nm to 1 mm, with a thickness from 2 nm to 5 μm beingmore typical.

The back contact structure of FIG. 5B also includes a back contacttransparent conductive material layer 68 located beneath layer 74. Theback contact transparent conductive material layer 68 of this embodimentcan include one of the conductive materials mentioned above for thefront contact transparent conductive material layer 32. The back contacttransparent conductive material layer 68 of this embodiment can also beformed utilizing one of the techniques mentioned above for the frontcontact transparent conductive material layer 32, and the thickness ofthe back contact transparent conductive material layer 68 can be withinthe range provided above for the front contact transparent conductivematerial layer 32. Similarly, a metal layer may be used instead of thetransparent conductive material layer 68. Typically, if layer 68 is atransparent conductive material, metal fingers 66 are required, while iflayer 68 is a metal, metal fingers 66 are not used. The usage of atransparent conductive material facilitates the bifacial operation ofthe solar cell, if desired.

Reference is now made to FIG. 5C which shows a localized back contactstructure that can be employed as the back contact 64 in thephotovoltaic device 50 shown in FIG. 4 Specifically, the back contactstructure shown in FIG. 5C includes a patterned dielectric material 76that is located beneath portions of the absorption layer 52. Thepatterned dielectric material 76 includes one of the dielectricmaterials mentioned for patterned dielectric material 36. The patterneddielectric material 76 can be processed using the technique mentionedabove in forming patterned dielectric material 36. The back contactstructure further includes back contact transparent conductive materiallayer 68 that is located beneath the patterned dielectric material 76and in gaps present between the patterned dielectric material 76. Asshown, portions of the back contact transparent conductive materiallayer 68 within the gaps between the patterned dielectric material 76are in direct contact with absorption layer 52.

FIG. 5D illustrates another back contact structure that can be employedas the back contact 64 shown in the photovoltaic device 50 of FIG. 4.FIG. 5D, which can be referred to as a localized back surface field backregion is similar to back contact structure shown in FIG. 5C except thathighly doped regions 78 are formed at the contact areas with thefingers. Highly doped regions 78 may be formed into the absorption layer52 (note the absorption layer 52 shown in FIG. 5D does not representanother absorption layer of the structure) by diffusion prior to formingthe back contact transparent conductive material layer 68. The highlydoped regions 78 have the same conductivity (i.e., first conductivity)as the absorption layer 52. However, the highly doped regions 78 have ahigh concentration of first conductivity type dopant than the absorptionlayer. When the highly doped regions 78 and the absorption layer 52 bothinclude an n-type dopant, the n-type dopant concentration in the highlydoped regions 78 can range from 10¹⁸ atoms/cm³ to 5×10²° atoms/cm³Likewise, when the highly doped regions 78 and the absorption layer 52both include a p-type dopant, the p-type dopant concentration in thehighly doped regions 78 can range from 10¹⁸ atoms/cm³ to 5×10²°atoms/cm³.

Referring back to FIG. 4, the photovoltaic device 50 also includes afront contact 54. In the front contact 54 of this embodiment of thepresent disclosure the intrinsic and/or doped hydrogenated amorphoussilicon (a-Si:H) layer(s) are replaced with an intrinsic and/or dopedlayer(s) of a semiconductor material having a lower valence band-offsetthan that of a:Si—H with c-Si, and/or higher activated doping level(s)than that of doped hydrogenated amorphous silicon.

The front contact 54 shown in FIG. 4 includes an optional front contactintrinsic amorphous semiconductor material layer 56 which is in directcontact with the top surface of the absorption layer 52. Again the term“intrinsic semiconductor” denotes a substantially pure semiconductorwithout any significant dopant species present. The number of chargecarriers in the intrinsic semiconductor is determined by the propertiesof the material itself instead of the amount of impurities, i.e.,dopants. Typically, in intrinsic semiconductors the number of excitedelectrons and the number of holes are equal (n=p). The front contactintrinsic amorphous semiconductor material layer 56 can serve topassivate the top surface of the absorption layer 52, and reduceelectron-hole recombination. The front contact intrinsic amorphoussemiconductor material layer 56 is typically, but not necessarily alwayshydrogenated. Typically, the front contact intrinsic amorphoussemiconductor material layer 56 is composed of intrinsic hydrogenatedamorphous silicon (i a-Si:H), hydrogenated amorphous Ge (a-Ge:H), orhydrogenated amorphous silicon germanium alloy (a-SiGe:H).

In some cases, the front contact intrinsic amorphous semiconductormaterial layer 56 comprises a same semiconductor material as theabsorption layer. In other embodiments, the front contact intrinsicamorphous semiconductor material layer 56 has a lower valenceband-offset than that of the absorption layer 52, and/or higheractivated doping level(s) than that of doped hydrogenated amorphoussilicon.

In cases in which the intrinsic layer includes SiGe, the Si (or Ge)content of the SiGe alloy may vary in a gradient fashion which, forinstance, can increase or decrease from a surface of layer 56 that isnearest to the absorption layer 52 to a surface of layer 56 that isfurthest from the absorption layer 52. The front contact intrinsicamorphous semiconductor material layer 56 can also include carbon,fluorine, deuterium, oxygen and/or nitrogen, which can be added duringor after formation of the intrinsic amorphous semiconductor materiallayer 56.

Typically, the thickness of the front contact intrinsic amorphoussemiconductor material layer 56 is from 3 nm to 30 nm, although lesserand greater thicknesses can also be employed.

The front contact intrinsic amorphous semiconductor material layer 56 isformed utilizing any chemical or physical growth process including anysemiconductor precursor source material. In some embodiments, theintrinsic hydrogenated semiconductor containing material used in forminglayer 56 is deposited in a process chamber containing a semiconductorprecursor source gas and a carrier gas which may include hydrogen.Hydrogen atoms in the precursor and/or carrier gas are incorporated intothe deposited material to form the front contact intrinsic amorphoussemiconductor material layer 56. The front contact intrinsic amorphoussemiconductor material layer 56 is optional, and may be omitted.

The front contact 54 shown in FIG. 4 also includes a front contactsemiconductor material layer 58 of a second conductivity type thatdiffers from the first conductivity type located either directly on thebottom surface of the absorption layer 52 or on a bottom surface oflayer 52. The front contact semiconductor material layer 58 employed inthis embodiment of the present disclosure has a lower valenceband-offset that that of hydrogenated amorphous silicon with crystallinesilicon, and/or a higher activated doping level than that of dopedhydrogenated amorphous silicon. In this embodiment and when theabsorption layer 52 is p-type, the front contact semiconductor materiallayer 58 has an n-type dopant concentration of from 10¹⁶ atoms/cm³ to10²¹ atoms/cm³, with an n-type dopant concentration within the frontcontact semiconductor material layer 58 from 10¹⁸ atoms/cm³ to 10²⁰atoms/cm³ being more typical. The doping efficiency (i.e., the ratio ofactivated dopant atoms to the total dopant atoms) in layer 58 typicallyranges from 0.1% to 20% although higher and lower doping efficienciesare possible. Typically, the doping efficiency is decreased byincreasing the dopant atom concentration. In embodiments, in which theabsorption layer 52 is n-type, the front contact semiconductor materiallayer 58 includes a p-type dopant concentration from 10¹⁶ atoms/cm³ to10²¹ atoms/cm³, with a p-type dopant concentration within the frontcontact semiconductor material layer 58 from 10¹⁸ atoms/cm³ to 10²⁰atoms/cm³ being more typical. The doping efficiency (i.e., the ratio ofactivated dopant atoms to the total dopant atoms) in layer 58 typicallyranges from 0.1% to 20% although higher and lower doping efficienciesare possible. Typically, the doping efficiency is decreased byincreasing the dopant atom concentration.

In one embodiment, and when the absorption layer comprises Si, the frontcontact semiconductor material layer 58 can be comprised of Ge or a SiGealloy. When a SiGe alloy is employed as front contact semiconductormaterial layer 58, the Si (or Ge) content may be constant throughout theentire thickness of the front contact semiconductor material layer 58.In other embodiments, the Si (or Ge) content of the SiGe alloy may varyin a gradient fashion which, for instance, can increase or decrease froma surface of the front contact semiconductor material layer 58 that isnearest to the absorption layer 52 to a surface of the front contactsemiconductor material layer 58 that is furthest from the absorptionlayer 52.

The front contact semiconductor material layer 58 can be amorphous,nanocrystalline (i.e., microcrystalline) or polycrystalline. In someembodiments, front contact semiconductor material layer 58 can behydrogenated. In other embodiments, the front contact semiconductormaterial layer 58 is not hydrogenated. The front contact semiconductormaterial layer 58 can also include carbon, fluorine, deuterium, oxygenand/or nitrogen, which can be added during or after formation of thefront contact semiconductor material layer 58.

The front contact semiconductor material layer 58 can be formedutilizing any physical or chemical growth process that is well known tothose skilled in the art. In one embodiment, the growth process includesan in-situ doped epitaxial growth process in which the dopant atom isintroduced with the semiconductor precursor source material, e.g., asilane, during the formation of the front contact semiconductor materiallayer 58. In another embodiment, a growth process is used to form anundoped semiconductor layer, and thereafter the dopant can be introducedusing one of ion implantation, gas phase doping, liquid solutionspray/mist doping, and/or out-diffusion of a dopant atom from anoverlying sacrificial dopant material layer that can be formed on theundoped semiconductor material, and removed after the out-diffusionprocess.

In some embodiments, the front contact semiconductor material layer 58can be comprised of a hydrogenated semiconductor material. Thehydrogenated semiconductor material that can be used as layer 58 can bedeposited in a process chamber containing a semiconductor precursorsource material gas and a carrier gas which may include hydrogen.Hydrogen atoms in the precursor and/or carrier gas are incorporated intothe deposited material to form a hydrogenated semiconductor-containingmaterial that can be used as layer 58.

The front contact 54 shown in FIG. 4 also includes a front contacttransparent conductive material layer 60 located atop layer 58. Thefront contact transparent conductive material layer 60 can include oneof the materials mentioned above for back contact transparent conductivematerial layer 20. The front contact transparent conductive materiallayer 60 can also be formed and having a thickness as mentioned alsoabove for back contact transparent conductive material layer 20.

The optional front contact metal fingers 62 can be formed atop the frontcontact transparent conductive material layer 60 utilizing one of thetechniques mentioned above.

FIGS. 6A-6D depicting other front contact structures that can be used inplace of the front contact 54 illustrated in FIG. 4. FIG. 6A provides afront contact structure that is similar to the front contact 54 shown inFIG. 4 except that front contact intrinsic amorphous semiconductormaterial layer 56 has been omitted from the structure.

FIG. 6B illustrates an embodiment in which a front contact hydrogenatedamorphous semiconductor material layer 57 of the second conductivitytype is positioned between layers 56 and 58. Thus, when the absorptionlayer 52 comprises a p-type semiconductor material, the front contacthydrogenated amorphous semiconductor material layer 57 comprises n-typedoping. In such an embodiment, the front contact hydrogenated amorphoussemiconductor material layer 57 has an n-type dopant concentrationranging from 10¹⁶ atoms/cm³ to 10²¹ atoms/cm³, with concentrationranging from 10¹⁸ atoms/cm³ to 10²⁰ atoms/cm³ being more typical. Thedoping efficiency (i.e., the ratio of activated dopant atoms to thetotal dopant atoms) in layer 57 typically ranges from 0.1% to 20%although higher and lower doping efficiencies are possible. Typically,the doping efficiency is decreased by increasing the dopant atomconcentration Likewise, when the absorption layer 52 comprises an n-typesemiconductor material, the front contact hydrogenated amorphoussemiconductor material layer 57 comprises p-type doping. In such anembodiment, the front contact hydrogenated amorphous semiconductormaterial layer 57 has a p-type dopant concentration ranging from 10¹⁶atoms/cm³ to 10²¹ atoms/cm³, with concentration ranging from 10¹⁸atoms/cm³ to 10²⁰ atoms/cm³ being more typical. The doping efficiency(i.e., the ratio of activated dopant atoms to the total dopant atoms) inlayer 57 typically ranges from 0.1% to 20% although higher and lowerdoping efficiencies are possible. Typically, the doping efficiency isdecreased by increasing the dopant atom concentration.

The front contact hydrogenated amorphous semiconductor material layer 57may or may not comprise the same semiconductor material as that of theabsorption layer 52. In one embodiment in which the absorption layer 52comprises Si, the front contact hydrogenated amorphous semiconductormaterial layer 57 comprises hydrogenated amorphous Si, and may containcarbon and/or germanium. The germanium and/or carbon concentration maybe constant or vary across the layer.

The front contact hydrogenated amorphous semiconductor material layer 57can be deposited in a process chamber containing a semiconductorprecursor source material gas, first conductivity type dopant and acarrier gas, which may contain hydrogen. Hydrogen atoms in the precursorand/or carrier gas are incorporated into the deposited material.

The thickness of the front contact hydrogenated amorphous semiconductormaterial layer 57 may vary. Typically, the front contact hydrogenatedamorphous semiconductor material layer 57 has a thickness from 2 nm to15 nm, although lesser and greater thicknesses can also be employed.

FIGS. 6C and 6D shows an embodiment in which front contact semiconductormaterial layer includes a first front contact semiconductor materiallayer 58′ and a second front contact semiconductor material layer 58″.In these embodiments, the first and second front contact semiconductormaterial layers 58′, 58″ are comprised of different semiconductormaterials that both have the lower valence band-offset than that ofhydrogenated amorphous silicon with the absorption layer 52, and/orhigher activated doping level(s) than that of doped hydrogenatedamorphous silicon. In one embodiment, the first front contactsemiconductor material layer 58′ is comprised of hydrogenated amorphousGe and the second front contact semiconductor material layer 58″ iscomprised of hydrogenated amorphous SiGe.

It is noted that in each of FIGS. 4, 6A, 6B, 6C and 6D, the collectionof holes at the front contact is improved by reducing the tunnelingbarrier for holes and/or enhancing the electric field at the emitterjunction. This is due to the lower valance band-offset between thesemiconductor material (s) within the front contact structure andcrystalline Si, and/or higher activated doping level(s) of thesemiconductor material(s) compared to that of doped hydrogenatedamorphous Si.

The higher level(s) of activated doping may be due to (i) higher dopingefficiency of at least one of the semiconductor materials within thefront contact structure, and/or (ii) “transfer doping” (also known as“modulation doping”) of at least one of the mentioned contact layermaterials.

The following examples are provided to illustrate some embodiments ofthe present disclosure and to also illustrate some advantages that canbe obtained therefrom.

Example 1

In a first exemplary example shown in FIG. 7A, the emitter diffusion(n⁺) layer was formed on the front side of a p-type crystalline Sisubstrate by phosphorous diffusion followed by a drive-in step usingstandard techniques which also formed a thermal oxide layer serving asanti-reflection coating (ARC). Metal fingers were formed after openingvias in the ARC using standard lithography. The back surface field wasformed by plasma-enhanced chemical vapor deposition (PECVD) of a 5 nm/5nm/15 nm i a-Si:H/p⁺/a-Si:H/p⁺/a-Ge:H stack at 200° C., followed byevaporation of an aluminum layer to form the back contact. In thisexemplary embodiment, the c-Si substrate was not textured. Theexperimental output characteristics of the photovoltaic device measuredunder a light intensity of 1 sun is plotted in FIG. 7B, showing an opencircuit voltage of 654 mV, a short circuit current density of 30.4mA/cm², and a fill-factor of 80.7%. In some embodiments, either or bothsurfaces of the c-Si may be textured to enhance light trapping. In someembodiments, other metal layers may be used instead of Al. In someembodiments, a transparent conductive material may be inserted betweenthe back contact metal and the a-Ge:H layer to enhance the reflection oflight from the back surface of c-Si absorbing layer. In someembodiments, a patterned metal layer may be used instead of a blanketmetal layer as the back contact. In some embodiments, the thickness ofthe i a-Si:H layer varies in the range of 0-10 nm, the thickness of thep⁺ a-Si:H layer varies in the range of 3-25 nm, and the thickness of thep⁺ a-Ge:H layer varies in the range of 3-50 nm. In some embodiments, thedeposition temperature of the PECVD stack may vary from room temperatureto 450° C. In some of these embodiments, some of the deposited layersmay be nano-crystalline, micro-crystalline or poly-crystalline. In someembodiments, a thermal annealing step in the range of 100-600° C. isperformed after or immediately after the deposition of the PECVD stackto recrystallize the a-Ge:H layer in the form of nano-crystalline,micro-crystalline, or poly-crystalline Ge, or improve the quality ofa-Ge:H or a-Si:H layers without recrystallization. The a-Si:H layers mayor may not recrystallize during this annealing step. In some embodimentswhere the back contact material is a metal such as Al, therecrystallization of the Ge layer may be facilitated by metal-inducedcrystallization. In an exemplary embodiment having the structuredescribed for FIG. 7A, and the experimental output characteristics ofFIG. 7B, annealing was performed in the range of 125-350° C., and as aresult, the open circuit voltage was improved from 654 mV to 670 mV.

Example 2

The front (emitter) contact in all the embodiments described in EXAMPLE1 may be formed by depositing an i a-Si:H/n⁺/a-Si:H PECVD stack insteadof diffusion, followed by the deposition of a transparent conductivematerial and metal fingers. An exemplary structure with the describedfront contact and a back contact the same as that explained for FIG. 7Ais illustrated in FIG. 7C. Open circuit voltages in the range of 703-763mV were experimentally measured under an intensity of 1 sun on thisexemplary structure after annealing in the range of 125-350° C. In otherembodiments, annealing temperatures may be in the range of 100-600° C.

While the present disclosure has been particularly shown and describedwith respect to various embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of forming a photovoltaic device comprising: providing anabsorption layer comprising a silicon-containing semiconductor layer ofa first conductivity type and having a top surface and a bottom surfacethat opposes said top surface; forming a front contact located on thetop surface of the absorption layer; and forming a back contact locatedon the bottom surface of the absorption layer, wherein said back contactcomprises at least one back contact semiconductor material layer of thefirst conductivity type and having a lower band-offset than that ofhydrogenated amorphous silicon with crystalline silicon, and/or a higherlevel of activated doping concentration compared to that of dopedhydrogenated amorphous Si.
 2. The method of claim 1, wherein saidproviding the absorption layer comprises selecting a p-type singlecrystalline silicon-containing material.
 3. The method of claim 1,wherein said providing the absorption layer comprises selecting ann-type single crystalline silicon-containing material.
 4. The method ofclaim 1, further comprising forming back metal fingers on a bottommostsurface of said back contact.
 5. The method of claim 1, furthercomprising forming front metal fingers on an uppermost surface of thefront contact.
 6. The method of claim 4, wherein said forming said backmetal fingers comprises screen printing or application of an etched orelectroformed metal pattern.
 7. The method of claim 5, wherein saidforming said front metal fingers comprises screen printing orapplication of an etched or electroformed metal pattern.
 8. A method offorming a photovoltaic device comprising: providing an absorption layercomprising a silicon-containing semiconductor layer of a firstconductivity type and having a top surface and a bottom surface thatopposes said top surface; forming a front contact located on the topsurface of the absorption layer, wherein said front contact comprises atleast one front contact semiconductor material layer of a secondconductivity type and having a lower band-offset than that ofhydrogenated amorphous silicon with crystalline silicon, and/or a higherlevel of activated doping concentration compared to that of dopedhydrogenated amorphous Si, and wherein said second conductivity type isopposite the first conductivity type; and forming a back contact locatedon the bottom surface of the absorption layer.
 9. The method of claim 8,wherein said providing the absorption layer comprises selecting a p-typesingle crystalline silicon-containing material.
 10. The method of claim8, wherein said providing the absorption layer comprises selecting ann-type single crystalline silicon-containing material.
 11. The method ofclaim 8, further comprising forming back metal fingers on a bottommostsurface of said back contact.
 12. The method of claim 8, furthercomprising forming front metal fingers on an uppermost surface of thefront contact.
 13. The method of claim 11, wherein said forming saidback metal fingers comprises screen printing or application of an etchedor electroformed metal pattern.
 14. The method of claim 12, wherein saidforming said front metal fingers comprises screen printing orapplication of an etched or electroformed metal pattern.